Modern complementary metal-oxide-semiconductor (“CMOS”) imagers often include some sort of feedback loop to automatically set the black level in the output. The analog voltage associated with true black may be obtained by reading “black reference pixels.” Black reference pixels are typically arrayed immediately next to the active image array. In frontside illuminated (“FSI”) image sensors, one of the metal layers within the frontside metal stack shields the reference pixels in order to block any incoming light. Circuitry within the FSI image sensor then sets the voltage output for the active pixels with reference to the output value from these black reference pixels. The black reference pixels are used to generate a low count value or a user specified set point value that will typically be displayed as black. Cameras are traditionally setup to a black level set point that is slightly greater than the read noise. Camera gain is then set to achieve a suitable image. Setting the proper black level is particularly important when working at very low signal levels or low ambient light environments. If the black level is set too low, dim objects will be clipped and not displayed. If the black level is set too high, image contrast will suffer.
FIG. 1A illustrates a conventional active pixel 100 of a FSI image sensor, while FIG. 1B illustrates a conventional black reference pixel 105 of a FSI image sensor. The frontside of imaging pixels 100 or 105 is the side of substrate 110 upon which the pixel circuitry is disposed and over which metal stack 115 for redistributing signals is formed. In active pixel 100, the metal layers (e.g., metal layer M1 and M2) are patterned in such a manner as to create an optical passage through which light incident on the frontside of active pixel 100 can reach the photosensitive photodiode (“PD”) region 120. In contrast, the optical passage of black reference pixel 105 is intentionally blocked and covered over with a metal layer M3.
When FSI image sensors are thinned via traditional methods, the backs of the black level reference pixels are exposed, as are the pixels in the active imaging array. Consequently, light and electrons incident upon the back of the die may induce signal noise in black reference pixel 105, thereby voiding its value as a black reference pixel. In order to regain the benefit of black reference pixels on backside thinned FSI image sensors, die shielding of incident electrons (or photons) is often used. One approach to such shielding is to deposit a metal layer over the backside of black reference pixel 105, as typically used on the front side. However, in order to maintain a low dark current either a special metalization is required for this backside metal layer or a dopant profile must be specified on the backside surface before the metal is deposited. In either case, this additional backside processing requires significant additional processes. This additional processing may involve a photolithography step to define the deposition area. The added masking and handling associated with photolithography can damage the sensitive frontside surface, thereby reducing yields.